The efferent lymph duct of the ovine prescapular lymph node was cannulated, and Salmonella abortusovis (SAO), a specific pathogen for sheep inducing abortion and mortality of newborn lambs, was inoculated by the subcutaneous route in this lymph node drained area. While the prescapular lymph node draining the inoculation site represented an efficient barrier for the vaccinal SAO Rv6 strain spreading, SAO 15/5 virulent bacteria were steadily detected in efferent lymph of infected sheep. The inoculation of the virulent strain of SAO induced a greater increase of the cell output than did the attenuated vaccinal strain, but proportions of blast cells appearing in the efferent lymph were similar in both cases. Flow cytometry analysis showed that B and T cell outputs were both increased during SAO infections, but while T cell subset proportions slightly decreased, B cell percentages significantly rose, and, at the peak response, almost all of the lymphoblast cells were activated B cells. Typical antibody profiles characteristic of a primary immune response were observed, and antibody titres were greater in the efferent lymph of animals inoculated with the virulent strain of SAO. Many of the cytokine mRNAs we investigated were steadily detected by RT-PCR in efferent lymph cells of control sheep, but frequencies of detection of IL-2, IFN gamma, IL-1 beta and TNF alpha mRNAs were augmented in efferent lymph cells following inoculation of both SAO virulent or vaccinal strains. IL-10 and IL-8 mRNAs could only be detected after a SAO inoculation, while detection of IL-4 mRNAs was increased only in efferent lymph cells from SAO virulent strain-infected sheep. The efferent lymph cannulation technique thus appeared a very powerful way to study the in vivo development of the immune response to SAO, in its natural host, the sheep.